1. Field of the Invention
This invention relates to improvements in electrical model and modeling techniques for modeling electrical circuits and associated mechanical apparatuses, and more particularly to improvements in electrical models and modeling techniques for use in modeling voice coil actuator/motor (VCM) circuits and apparatuses of the type used in mass storage devices, or the like.
2. Relevant Background
A well-known hard disk drive assembly (HDA) is a typical mass data storage device of the type to which the invention pertains. Generally the HDA includes one or more rotating disks that carry a magnetic media to which data is written, and from which previously written data is read. The data is written to and read from the disk by a magnetic head or transducer that is a part of a voice coil motor (VCM) assembly, which moves the head to desired locations at which data is to be written or read.
A portion of a typical HDA 5 is shown in FIG. 1. The HDA 5 includes a VCM apparatus 10, which is shown in an exploded view in FIG. 1, in conjunction with a plurality of rotating disks 12. The VCM assembly 10 includes an arm 14 that is pivoted about a bearing point 16 to move the head or data transducer 18 radially inwardly and outwardly within the stack of data disks 12.
The outboard end of the arm 14 carries a coil 20 that is selectively energized by currents from VCM positioning circuitry 22. The outwardly extending end 24 of the arm 14 is located between two horizontal magnets 26 and 28, which are mounted to base plates 30 and 32. The base plates 30 and 32 and magnets 26 and 28 are spaced apart by spacers (not shown) to allow the arm portion 24 to freely swing between the magnets 26 and 28. Thus, as the currents from the VCM positioning circuitry 22 are applied to the coil 20, magnetic fields are altered by the current induced field of coil 20 that can precisely position the head 18 at a desired location under control of the VCM positioning circuitry 22. The plates 30 and 32, spacers, and magnets 26 and 28 are securely fastened to the base plate 34. The cover plate 35 is on the top of the base plate 32. The two plates 32 and 35 may physically touch or barely touch each other.
When the apparatus 5 is powered down, typically the head mechanism is moved to a position at which the head or transducer 18 is xe2x80x9cparkedxe2x80x9d or xe2x80x9clandedxe2x80x9d, generally at an outside location along the outer radius of the disk assembly 12. In order to properly move the heads to the park position, generally a driving current is applied to the coil 20 that is of sufficient magnitude to bring the head assembly to the park position. However, it will be appreciated that if the head mechanism is overdriven, the delicate head mechanism and other parts of the disk assembly may sustain damage. On other hand, if the head is underdriven, the head mechanism may not reach the park position, which may result in loss of the air bearing between the head and disk surface, which may also cause damage both to the head mechanism and to the underlying magnetic media of the disk assembly 12. As a result, it can be seen that in order to properly design the head positioning circuitry to apply correct driving currents to the voice coil, it is important to properly characterize the head assembly so that proper driving currents for various conditions can be determined.
One technique controlling the VCM is shown in U.S. patent application Ser. No. 09/388,508, filed Sep. 1, 1999 (attorney docket number TI-29097), incorporated herein by reference. One technique measuring the BEMF of the coil of the actuator used in said application Ser. No. 09/388,508 is shown in U.S. patent application Ser. No. 09/193,803, filed Nov. 17, 1998 (attorney docket number TI-26417), incorporated herein by reference.
In the past, in order to characterize the VCM assembly, a model was constructed of the electrical characteristics of the VCM. A typical model 40 of a VCM that has been used is shown in FIG. 2, to which reference is now made. The model 40 has a resistor 42, an inductor 44, and a voltage source 46 connected in series between input nodes 48 and 49. The voltage source 46 represents the back emf (BEMF), which has a value almost directly proportional to the speed of the VCM.
However, the model 40 has been found to only roughly represent the characteristics of a VCM. In particular, the model 40 may encounter deviations from the physical operations of the VCM, especially when the terminals 48 and 50 are in a floating stage for measurement of BEMF. (The BEMF is used in determining the velocity of the head, particularly during parking operations described above.) More particularly, it has been found that when the power supply voltage across the input terminals of the VCM is cut off, theoretically the VCM should settle to its BEMF rapidly as soon as the flyback current dies out. In practice, this has been found not to be the case. In fact, there has been found to almost always be a slow transient associated with the voltage across the terminals resulting in a longer settling time than would be predicted by the model 40.
What is needed, therefore, is a method and circuit to accurately model a VCM of a mass data storage device that more accurately represents the physical VCM characteristics.
Thus, according to a broad aspect of the invention, a circuit model for use in analyzing a VCM is presented. The circuit model has a first current loop having elements to model a VCM coil and magnetic field. At least one current loop is inductively coupled to the first current loop to model eddy current effects created in metallic parts of the VCM in proximity to the VCM coil.
According to another broad aspect of the invention, a circuit model for use in analyzing a VCM circuit is presented. The model has a model of a first resistor, a model of a first inductor, a model of a second Inductor and a model of a voltage source, in series between models of first and second input nodes. The model of the second inductor represents a VCM mutual inductance, the model of the first inductor represents a winding leakage inductance of the VCM inductor, the model of the first resistor represents a VCM coil resistance, and the model of a voltage source represents a BEMF voltage generated by actuator speed.
A model of an input capacitor is provided across the models of the first and second input terminals. The model of the input capacitor represents an equivalent capacitance of the VCM inductor. Models of third and fourth inductors and a model of a second resistor are provided, connected in a first series loop. The model of the third inductor is in transformer relationship to the model of the second inductor. The model of the third inductor represents an inductance of a top VCM magnetic plate, the model of the fourth inductor represents the leakage inductance of the top VCM magnetic plate, and the model of a second resistor represents a resistance of the top VCM magnetic plate. Models of fifth and sixth inductors and a model of a third resistor are connected in a second series loop. The model of the fifth inductor is in transformer relationship to the model of the second inductor and the first and second series loops are interconnected. The model of the fifth inductor represents an inductance of the bottom magnetic plate, the model of a sixth inductor represents the leakage inductance of the bottom VCM magnetic plate, and the model of the third resistor represents the resistance of the bottom VCM magnetic plate.
If needed, a fourth resistor may be provided in parallel with the model of the second inductor. The model of the fourth resistor represents the resistance in parallel with the VCM mutual inductor. Additionally, if needed, models of second and third capacitors may be respectively connected between the model of the second inductor and the first and second loops. The models of the second and third capacitors respectively represent an equivalent capacitance between the VCM coil and the top and bottom VCM magnet plates.
In accordance with another broad aspect of the invention, acts used in a method for analyzing a VCM circuit include determining eddy currents existing in a coil of a VCM, using the eddy currents to determine effective inductances that would produce the eddy currents, and including the inductances in a model of the VCM for use in analyzing the VCM.